The invention relates to a control arrangement which is used to supply at least two hydraulic consumers with pressure medium. The invention also relates to a pressure differential valve which is used in particular in said control arrangement.
A hydraulic control arrangement of this type is known, for example from EP 0 566 449 A1. This document relates to a hydraulic control arrangement using the load-sensing principle, in which a variable-displacement pump is set, as a function of the highest load pressure of the actuated hydraulic consumers, in each case in such a way that the feed pressure is higher than the highest load pressure by a defined pressure difference. The pressure medium flows to the two hydraulic consumers via two adjustable meter-in variable restrictors, a first of which is arranged between a pump line leading from the variable-adjustment pump and a first hydraulic consumer, and the second of which is arranged between the pump line and the second hydraulic consumer. The pressure compensators connected downstream of the meter-in variable restrictors mean that, if a sufficient quantity of pressure medium is supplied, there is a defined pressure difference across the meter-in variable restrictors irrespective of the load pressures of the hydraulic consumers, so that the quantity of pressure medium flowing to one hydraulic consumer is only dependent on the opening cross section of the meter-in variable restrictor in question. If a meter-in variable restrictor is opened further, it is inevitable that a greater quantity of pressure medium will flow through it, in order to generate the defined pressure difference.
The variable-displacement pump is in each case adjusted in such a way that it supplies the quantity of pressure medium which is required. This is therefore known as control based on the required flow.
The pressure compensators which follow the meter-in variable restrictors are acted upon in the opening direction by the pressure downstream of the respective meter-in variable restrictor and in the closing direction by a control pressure which prevails in a rear control space and which usually corresponds to the highest load pressure of all the hydraulic consumers supplied by the same hydraulic pump. If, in the event of simultaneous actuation of a plurality of hydraulic consumers, the meter-in variable restrictors are opened so wide that the quantity of pressure medium supplied by the hydraulic pump, which has been moved all the way to its stop, is lower than the total quantity of pressure medium required, the quantities of pressure medium flowing to the individual hydraulic consumers are reduced in equal proportions irrespective of the prevailing load pressure of the hydraulic consumers. This is therefore referred to as control with load-independent flow distribution (LIFD control). Hydraulic consumers which are controlled in this way are known as LIFD consumers for short. Since, with LIFD control, the highest load pressure is also sensed and a feed pressure which lies above the highest load pressure by a defined pressure difference is generated by the pressure medium source, LIFD control is a special case of a load-sensing control (LS control).
There is no load-independent flow distribution in the case of a plurality of hydraulic consumers to which pressure medium flows in each case via a meter-in variable restrictor with upstream pressure compensator which is acted upon in the closing direction only by the pressure upstream of the meter-in variable restrictor and in the opening direction only by the load pressure of the corresponding hydraulic consumer and by a compression spring. This is a case of a simple LS control and an LS consumer. A control set-up of this type is known, for example, from DE 197 14 141 A1. In the event of simultaneous actuation of a plurality of hydraulic consumers and of insufficient quantities of pressure medium being supplied by the variable-displacement pump, in this case only the quantity of pressure medium flowing to the hydraulic consumer with the highest load pressure is reduced.
An advantage of LS control with pressure compensators connected upstream of the meter-in variable restrictors compared to LS control with pressure compensators connected downstream of the meter-in variable restrictors, however, is that, in the event of an excess quantity being supplied for a brief time by the variable-adjustment pump and an associated rise in the feed pressure, the upstream pressure compensators, by reducing their opening cross section, do not allow any increase in the pressure difference across the meter-in variable restrictors, so that no further pressure medium flows across the meter-in variable restrictors and the speed of the hydraulic consumers is not changed. The excess quantity flows back to a tank via a pressure-limiting valve. In the case of a control set-up with pressure compensators connected downstream of the meter-in variable restrictors, by contrast, the excess quantity is passed through to the hydraulic consumers.
Depending on whether the user attaches more importance to a load-independent flow distribution or to preventing excess quantities from flowing to the hydraulic consumers, he will select an LIFD control or an LS control. This has hitherto been a drawback for the manufacturers of hydraulic components, since they have to offer control blocks for both LIFD control set-ups and for LS control set-ups. These differ considerably, since very divergent structures are required depending on whether a pressure compensator is connected upstream or downstream of the corresponding meter-in variable restrictor.
By contrast, the invention is based on the objective of providing a hydraulic control arrangement which has the features of the introductory-mentioned type, i.e. in which in particular pressure compensators are connected downstream of meter-in variable restrictors, in such a way that the flow of excess quantities to the hydraulic consumers is prevented.
The desired object is achieved, according to the invention, in a hydraulic control arrangement of the generic type, wherein the control pistons of the pressure compensators can be acted on in the closing direction by a control pressure which is present in a rear control space, is derived from the feed pressure prevailing in the feed line with the aid of a valve device and changes with the feed pressure. While in the known hydraulic control arrangement with pressure compensators connected downstream of the meter-in variable restrictors these compensators are acted on in the rear control space by the highest load pressure, on which the delivery quantity of the variable-displacement pump has no influence, in a control arrangement according to the invention the control pressure which is present in the rear control space is derived from the feed pressure and changes with the latter. Therefore, if the feed pressure rises on account of a delivery quantity from the variable-displacement pump rising beyond demand, the control pressure also rises. The control pistons of the pressure compensators are moved accordingly in the closing direction, so that the pressure downstream of the meter-in variable restrictors also rises and the pressure difference across the meter-in variable restrictors does not change. However, a constant pressure difference across a meter-in variable restrictor also means a constant quantity of pressure medium flowing across the meter-in variable restrictor. Therefore, while maintaining the basic arrangement of meter-in variable restrictor and downstream pressure compensator, and therefore without fundamental changes to a control block, the same control performance as in the case of a control set-up with pressure compensators connected upstream of the meter-in variable restrictors, i.e. control blocks of altogether different construction, is achieved with minor modifications.
Therefore, the difference between the feed pressure and the control pressure, when the variable-displacement pump has not been displaced as far as its stop, i.e. when there is a sufficient quantity of pressure medium, is preferably no greater than between the feed pressure and the highest load pressure. This is because if the pressure difference were greater, the quantity of pressure medium flowing to one hydraulic consumer would depend on whether the load pressure of this hydraulic consumer is higher or lower than the control pressure. The control pressure is preferably slightly higher than the highest load pressure, so that on the one hand there are no unnecessary throttling losses at the pressure compensators, but on the other hand in each case the pressure compensator assigned to the hydraulic consumer with the highest load pressure is still within the control range.
In principle, it is conceivable for the pressure difference between the feed line and a rear control space at a pressure compensator to be produced by connecting a nozzle between the feed line and the control space and by connecting a flow-regulating valve between the control space and a tank. In each case a defined quantity of control fluid would flow out of the control space to the tank via the flow-regulating valve. This quantity of control fluid would flow to the control space via the nozzle. Therefore, there would be a constant pressure gradient across the nozzle. However, the quantity of pressure medium flowing via a nozzle is highly dependent on the viscosity of the pressure medium. It therefore appears more appropriate to use a pressure differential valve, an inlet of which is connected to the feed line and an outlet of which is connected to the rear control space of a pressure compensator, instead of a nozzle. According to a feature of the invention, the pressure differential valve is preferably set to a fixed pressure difference and has a movable valve member which is acted on by the feed pressure for the purpose of opening fluid communication between the feed line and the control space at the pressure compensator and is acted on by the control pressure and by a spring for the purpose of closing this communication.
The invention provides a particularly preferred configuration, according to which the rear control spaces of a plurality of pressure compensators are directly connected to one another, so that the same control pressure prevails in these control spaces. Therefore, only one valve device for deriving the control pressure from the feed pressure is required for these pressure compensators. In a particularly advantageous configuration, the control arrangement has a load signaling line, to which the highest load pressure of the hydraulic consumers actuated in each case is input via selection valves, and a valve which opens up fluid communication from the load signaling line to the rear control space of at least one pressure compensator when the difference between the feed pressure and the highest load pressure falls below a defined level. In this way, in the case of undersaturation, i.e. in the event of insufficient pressure medium being delivered by the variable-displacement pump, the result is a load-independent flow distribution between the hydraulic consumers, the control space of the pressure compensators of which is connected to the load signaling line.
If the supply of pressure medium to one hydraulic consumer is to be prioritized over the supply to another hydraulic consumer in the event of undersaturation, this is advantageously achieved by a configuration, wherein the rear control space at the pressure compensator of the hydraulic consumer which is to be supplied with pressure medium as a priority is then separate from the control spaces at the pressure compensators of the other hydraulic consumers. The control pressure in this priority consumer is derived from the feed pressure via a further valve device. Moreover, there is a priority valve, by means of which, in order to maintain a desired pressure difference across the meter-in variable restrictor arranged upstream of the pressure compensator of the prioritized hydraulic consumer, and therefore to maintain a sufficient supply of pressure medium to the prioritized hydraulic consumer, in the event of a quantity of medium delivered by the variable-displacement pump not meeting demand, the control pressure in the rear control space of the other hydraulic consumers is raised to above the control pressure in the case of saturation. The priority valve preferably has a first port, which is connected to the feed line, and a second port, which is connected to the rear control spaces of the pressure compensators assigned to the hydraulic consumers which are not prioritized, and has a valve member, which, in the direction of opening the connection between the first port and the second port, can be acted on by the pressure prevailing in a line section downstream of the meter-in variable restrictor assigned to the prioritized hydraulic consumer and by an additional force, and, in the direction of closing the connection between the first port and the second port, can be acted on by the feed pressure. Downstream of the meter-in variable restrictor, a control space of the priority valve may be connected to the line section upstream or downstream of the pressure compensator, since the priority valve comes into action when the pressure compensator is completely open and because the same pressure, namely the load pressure of the prioritized hydraulic consumer, then prevails upstream and downstream of the pressure compensator.
A further object of the invention is to provide a pressure differential valve which is used in particular to derive a control pressure for a pressure compensator from the feed pressure in a control arrangement, which is of particularly small structure, so that it can readily be inserted into a control block.
A pressure differential valve of this type is obtained by features of the invention.
Advantageous configurations of a pressure differential valve of this type are also provided by the invention.
A further object of the invention is to provide a pressure differential valve which is used in particular to derive a control pressure for a pressure compensator from the feed pressure in a control arrangement in accordance with one of patent claims 1 to 9 and which is of particularly small structure, so that it can readily be inserted into a control block.
A pressure differential valve of this type is obtained by means of the features given in the defining part of patent claim 10.
Advantageous configurations of a pressure differential valve of this type are given in patent claims 11 to 13.